There are many medical circumstances in which an increase in the supply of blood to living tissue is indicated. These include: burns and wound healing, in which the incorporation of angiogenic factors into artificial skin may facilitate the formation of blood vessels in the healing wound bed and also reduce the risk of infection; cardiovascular disease, in which repair of anginal or ischemic cardiac tissue can be effected by the ingrowth of new blood vessels; stroke, where increased blood supply to the brain can reduce the risk of transient ischemic attack and/or cerebral arterial deficiency; and peripheral vascular disease, in which blood flow in the extremities is diminished. In each case, it is believed that the growth of new blood vessels will increase the volume of blood circulating through the tissue in question, and correspondingly increase the amount of oxygen and nutrients made available to that tissue.
One common cause of decreased blood flow is atherosclerosis. Atherosclerosis affects the blood vessels, including those of the heart, and is a major cause of cardiovascular disease, stroke and peripheral vascular disease. This disease may have its beginnings early in life and is initially characterized by a thickening of the arterial walls. This thickening typically comprises an accumulation of fat, fibrin, cellular debris and calcium. The resultant narrowing of the lumen of the afflicted vessel is called stenosis. Stenosis impedes and reduces blood flow. Hypertension and dysfunction of the organ or area of the body that suffers the reduced blood flow can result. As the buildup on the inner wall of a vessel thickens, the vessel wall loses the ability to expand and contract. Also, the vessel loses its viability and becomes weakened and susceptible to bulging, also known as aneurysm. In the presence of hypertension or elevated blood pressure, aneurysms will frequently dissect and ultimately rupture.
Small vessels, such as the arteries that supply blood to the heart, legs, intestines and other areas of the body, are particularly susceptible to atherosclerotic narrowing. When an artery in the leg or intestine is affected, the resultant loss of blood supply to the leg or segment of the intestine may result in gangrene. Atherosclerotic narrowing of one or more of the coronary arteries limits, and in some instances prevents altogether, blood flow to portions of the heart muscle. Depending upon the severity of the occlusion and its location within the coronary circulation system, pain, cardiac dysfunction or death may result. Because the consequences of blocked arteries are so serious, reliable treatments are highly desirable.
In many instances, it is possible to correct aneurysms and stenosis of major arteries using plastic reconstruction that does not require any synthetic graft or patch materials. In other instances, such as where the disease is extensive and the vessel is no longer competent, the blocked or weakened portion of the vessel is usually replaced with a graft. In such case, the affected vessel section is transected and removed and a synthetic patch, conduit or graft is sewn into its place. These types of procedures, including coronary artery bypass grafting (CABG) and percutaneous transluminal coronary angioplasty (PTCA), are routinely performed for the purpose of alleviating ischemia.
Nevertheless, coronary artery disease alone is responsible for approximately 550,000 deaths each year in the United States. Peripheral vascular disease results in lower limb amputation in about 150,000 patients each year, with a subsequent mortality rate of 40% within two years of amputation. Some of the difficulty in treating arterial occlusions may lie in the fact that each of the aforementioned surgical procedures is associated with a certain incidence of restenosis and may not be appropriate in certain instances. This is particularly true when the patient is elderly or has undergone a previous CABG or PTCA procedure. Accordingly, in such cases a less invasive technique would be preferred. In particular, it would be advantageous to be able to stimulate the surrounding tissue to produce for itself new vessels that would accommodate the volume of blood flow that has been occluded and thus compensate for the occluded vessels.
Angiogenic, or “vessel-growing” polypeptide growth factors in general have been the subject of much research. Such compositions offer the promise of a non-invasive, non-surgical treatment of arterial occlusion in a variety of situations, including those noted above. However, one major challenge facing the development of physiological treatments based on these materials is the lack of suitable solvents or carriers for the administration of growth factors to living patients.
A number of solvents have been used as carriers for particular applications within the pharmaceutical arts. Thus, for example, U.S. Pat. No. 5,981,489 (Stevenson et al.) discloses a number of non-aqueous protic formulations of peptides. Among the solvents mentioned therein are polyethylene glycols (PEGs), propylene glycol (PG), polyvinylpyrrolidone (PVP), methoxypropylene glycol (MPEG), glycerol, and glycofurol.
However, many solvents that adequately solubilize some pharmaceutical preparations and biological materials do not adequately solubilize polypeptide growth factors. Consequently, treatments utilizing such solvents may not induce an angiogenic response, even if the particular growth factor being used is capable of inducing such a response when properly delivered to the body of the patient. Other solvents tend to interact with, denature, induce crosslinking or cause other undesirable reactions in polypeptide growth factors, thus causing them to coagulate or precipitate, or otherwise rendering them pharmaceutically inactive or unsuitable. This is especially true if the solvent medium does not afford an acceptable pH range to the polypeptide growth factor (e.g., if it is too basic). Still other potentially useful solvents do not form stable solutions with growth factors, and hence cannot be used to make formulations having acceptable shelf stability.
Many potentially useful solvents for growth factors are also physiologically unsuitable. For example, investigators have used dilute solutions of hydrochloric acid (1 to 10 mM) to solubilize certain growth factors. See, e.g., R. J. Laham, M. Rezaee, M. Post, D. Novicki, F. W. Sallke, J. D. Pearlman, M. Simmons and D. Hung, “Intrapericardial Delivery of Fibroblast Growth Factor-2 Indices Neovascularization in a Procine Model of Chronic Myocardial Ischemia”, J. Pharmocolo. Exp. Ther. 292(2), 795–802 (2000); N. Yamamoto, T. Kohmoto, W. Roethy, A. Gu, C. DeRosa, L. E. Rabbani, C. R. Smith, and D. Burkhoff, “Histological Evidence That Basic Fibroblast Growth Factor Enhances the Angiogenic Effects of Transmyocardial Laser Revascularization”, J. Pharmocolo. Exp. Ther. 95(1), 55–63 (2000). However, for clinical applications, acid solvents such as dilute mineral acids are unsuitable because their use tends to cause cell damage or death in the proximity of the site of administration. On the other hand, saline solutions and neutral buffered salt solutions, which are more biologically compatible than mineral acids and which are used as solvents in some pharmaceutical formulations, cause many growth factor proteins to precipitate or become denatured, thus decreasing their bioavailability or effectiveness in the tissue.
The selection of a suitable solvent for growth factors is further complicated by other considerations. For example, even if a solvent adequately solubilizes a growth factor and does not cause it to become denatured or otherwise adversely affected, the solvent may nonetheless interfere with elution of the growth factor from the solvent medium, thereby reducing its efficacy. This may be the case, for example, if the solvent is too viscous, or bonds to the growth factor (e.g., through hydrogen bonding or dipole-dipole interactions) too strongly.
There is thus a need in the art for a solvent or carrier that solubilizes polypeptide growth factors sufficiently to render them pharmacologically useful, that does not cause them to coagulate, become denatured, or undergo crosslinking or other reactions that would adversely affect their pharmaceutical activity, that exhibits good shelf stability, that does not cause significant injury or damage to cells at the site of administration, and that allows the growth factor to properly elute from the solvent medium after administration to living tissues. These and other needs are addressed by the present invention, as hereinafter described.